A variety of prosthesis configurations have been proposed and used to implant prostheses in bone. See, for example, Noiles, U.S. Pat. No. 4,219,893 (see FIGS. 7-9); Meyer, U.S. Pat. No. 4,549,319; Noiles, U.S. Pat. No. 4,790,852; Penenberg et al., U.S. Pat. No. 4,808,185; Noiles, U.S. Pat. No. 4,846,839; Luman, U.S. Pat. No. 5,002,578; and the C-2 Conical Collar.TM. Hip System of the Kirschner Medical Corporation, Timonium, Md., 21093.
In overview, one or more cavities are prepared at the end of the bone for receiving the prosthesis. The prosthesis is then inserted into the prepared cavity and held in place by a mechanical fit or in some cases by bone cement. The initial fit between the prosthesis and the bone is critical to the long term success of the prosthesis especially with a mechanical fit.
Two fundamental criteria which a prosthesis and its cavity should meet are: (1) proper transfer of force from the prosthesis to the bone (see Meyer, U.S. Pat. No. 4,549,319); and (2) ready formation of the cavity so as to achieve an accurate fit with the prosthesis (see Noiles, U.S. Pat. No. 4,790,852).
With regard to the first criterium, force needs to be effectively transferred from the prosthesis to the cortical (hard/strong) bone and, in particular, to the cortical bone at the extreme end of the bone, e.g., the proximal end of the femur in the case of the femoral component of a hip joint, the distal end of the femur in the case of the femoral component of a knee joint, and the proximal end of the tibia in the case of the tibial component of a knee joint.
In the case of knee joints, instead of directly transferring force to the cortical bone, it is also common practice to have the prosthesis abut primarily cancellous bone and have the cancellous bone transmit force to the cortical bone. In such cases, the cancellous bone must have sufficient structural strength to sustain the loads imposed on it.
The application of forces of physiological magnitudes to bone fosters bone growth in the region where the forces are applied. The transfer of force to the cortical bone at the extreme end of the bone thus leads to bone growth in this critical region, If the end of the bone is not loaded, bone resorption can occur in this region. This leads to a diminished amount of bone which is undesirable in its own right and is particularly troublesome should the prosthesis fail and need to be replaced.
With regard to the second criterium, for repeatable success, the cavity for the prosthesis must be created in a precise and reproducible manner. The cavity preparation procedure preferably accommodates the anatomical variation between patients. Moreover, the surgical site does not favor complex procedures for preparing a cavity for implantation of a prosthesis in bone.
Prior techniques have achieved these two criteria to greater or lesser extents. Loading at the extreme end of the bone has not been a natural consequence of the prosthesis' configuration in many cases. With regard to bone preparation, many prostheses require cavities whose configurations do not lend themselves to precise machining. The loading and bone preparation criteria have often led to compromises regarding other desirable criteria. For example, the ability to provide a one piece prosthesis which can be used in either the right or left hand bones of the patient has been difficult with prior prostheses.